Offshore oil and gas production is being pushed both into deeper waters, and pushed to extend further from or between operational facilities. As such, ‘pipe-in-pipe’ pipelines, also known as PIP pipelines, are being developed with more and more accessories. PIP pipelines are well known in the art, and generally comprise an outer pipe and at least one, usually one, concentric or co-axial inner pipe. The inner pipe is used to transport or convey fluids such as hydrocarbons, in particular oil and gas, between two or more static and/or moveable locations, and can also be termed the flowline. This includes conveying fluids between vessels, or locations at or near a seabed such as an oil head, in particular a remote oil head, to an underwater facility and/or to a riser towards sea level, and/or directly to an onshore facility.
The outer pipes forming the pipeline generally provide protection, as well as an annular space between the inner and outer pipes for various purposes. These can include thermal insulation, as well as a number of accessories having a specific function. For example, there can be transverse bulkheads for transferring mechanical loads between the outer pipe and the inner pipes. Buckle arrestors prevent the provocation of radial buckles, such as that shown in U.S. Pat. No. 6,701,967 which describes a double-walled pipe of the reelable type. Waterstops avoid the ingress of water within a compartment defined between two waterstops.
More particularly, the annular space can also be for the provision of one or more cables extending beyond the PIP pipeline. Such cables can include fibre optic cables for the transmission of signals and other information, as well as heating cables designed to warm the inner pipe to help avoid the risk of hydrate plugging and wax formation in oil and gas pipelines and flow lines.
As is known, the resistance to flow of liquid products such as oil increases as temperature decreases, and the formation of hydrates and waxes in oil and gas pipelines can block such pipelines. Conventional methods of remediating the problem of hydrate or wax plugs can involve the use of chemical agents, blowdowns and pigging operations, as well as heating of the pipeline and the provision of thermal insulation.
Chemical agents can also be injected into the fluid being conveyed by the inner pipeline in order to enhance the recovery of the hydrocarbons, particularly over a long distance, through a relatively cold region, and/or to a surface installation. For example, methanol can be injected to avoid the formation of hydrates that would otherwise plug the pipeline.
The injection of chemical agents requires an injection port. Conventionally, this is provided by a “T-piece” in the PIP pipeline at a required location. Conventionally, a T-piece is a short section of a co-axial pipe having inner and outer pipe sections connected with a transverse section, which transverse section includes a hole or tube for the injection of the chemical agent into the inner pipeline. Such a conventional T-piece is shown in FIGS. 1a and 1b of the accompanying drawings.
Meanwhile, there are two common methods of laying underwater or marine pipelines. The ‘stove piping method’ involves assembling pipe stalks on a marine pipe-laying vessel, and then welding each one as the laying progresses. In the ‘reeled lay method’, the pipeline is assembled onshore from a number of stalks or sections, spooled onto a large reel, sometimes also termed a storage reel or drum, and then unwound from the reel as the pipe is being laid. Once offshore, the pipeline is unwound from the reel and is directly available for laying through a lay ramp, with no section-welding being required during the offshore operation.
The reeled lay method is faster than the stove piping method, such that it is preferred where possible. Thus, for the introduction of a conventional T-piece in a reeled pipeline during the reeled lay method, the reeled pipeline is unwound from the reel and cut on the lay ramp. Lay ramps are well known in the art, and generally include one or more clamps able to hold the pipeline immediately prior to its entry into the marine environment.
As depicted in FIGS. 1a and 1b herewith, the un-reeled pipeline 2 is cut, to allow the introduction of a conventional T-piece 4. The inner pipe 6 of the T-piece 4 is then welded via welds 5 to the inner pipe 7 of the pipeline 2 first, and then two outer shells 8 are provided to create the connection and continuation of the outer pipe 10 of the conventional T-piece 4, and the outer pipe 12 of the pipeline 2. The conventional T-piece 4 provides an injection port 14 for the introduction of methanol (MeOH) or other chemical agents as discussed above. Once the welding is complete as shown in FIG. 1b herewith, the laying of the pipeline 2 can continue.
Whilst the method and operation shown in FIGS. 1a and 1b are known in the art, they are not carried out on PIP pipelines having cables extending along the annular space between the inner and outer pipes, as this would also require the cutting and hopeful reconnection of all the cables. Not only have such cables been particularly fitted during installation onshore, and may not be readily accessible again for reworking offshore, but assurance is also needed concerning the integrity of the reconnections, especially the optimal continuation of the cable functions if cut and reconnected, which may include fibre optic signal transmissions. Thus, the cutting of such cables is not recommended in the art.
Also, the nature and shape of the conventional T-piece as shown in FIGS. 1a and 1b means that it cannot be added into the pipeline as part of the assembling process onshore for two reasons. Firstly, the injection port extends beyond the outer circumference of the pipeline, which does not allow it to be properly reeled onto a storage reel. Secondly, it is not able to accommodate cables.
It is an object of the present invention to provide an improved T-piece, through the provision of a T-piece preformer, a PIP pipeline comprising said T-piece preformer and subsequently said T-piece, as well as to provide methods of installing the T-piece preformer, and of providing the T-piece in the laying pipeline, as well as laying such as pipeline.